of Edinburgh , Session 1881-82. 
833 
Reaction. 
E.M.F. of 
Metal. 
Heats of Com- 
bination, calcu- 
lated from 
E.M.F. 
Heats of Com- 
bination, from 
Naumann’s 
Table. * 
Zn + I 2 =ZnI 2 
1*324 volts. 
924 heat units. 
929 heat units. 
Cu 2 + 1 2 — CU 2 I 2 
•657 „ 
235 „ 
256 „ 
Ag 2 + I 2 =Ag 2 I 2 . 
*624 „ 
131 „ 
127 „ 
Pb + I 2 = PbI 2 
*861 „ 
189 „ 
191 „ 
Fe + I 2 = FeI 2 . 
•667 „ 
523 „ 
851 „ 
In order to test the effect of temperature on the E.M.F. of the cell, 
the E.M.F. of zinc in an iodine cell was measured when the cell was 
at 11° C., and when it was at 27° C. The E.M.F. in both cases was 
the same. 
The deflection given by silver wire and silver electrolytically 
deposited from pure nitrate of silver is the same. 
Iron wire with a blue tarnish on it gives a considerably less 
deflection than clean iron. 
The E.M.F. given by alloys seemed to us to be of considerable 
interest. We supposed that they would give the E.M.F. of their 
most positive constituent. This, however, is not always the case. 
Solder gives a deflection of 40 (lead = 40, tin = 32), but brass gives 
the same deflection as copper, namely, 33. A confirmation of this 
is the fact that brass and copper give no E.M.F. in dilute sulphuric 
acid. 
After having made these experiments with the iodine cell, we 
made a chlorine cell by filling the porous pot with a freshly pre- 
pared solution of chlorine. The results with the chlorine cell do 
not agree well with the results found in the cuprous chloride cell, 
and the heats of combination calculated from the electromotive 
forces in the chlorine cell must be regarded as very rough approxi- 
mations. Two values are worth mentioning. The heat of combina- 
tion of 1 gramme of zinc with chlorine, calculated from its E.M.F. 
* At the time this paper was read we had not seen Naumann’s Thermochemie 
(published 1882). The above values have therefore been inserted since then. 
They are much nearer our own than the values given in the older books. 
